It is often necessary or desirable to provide for heat flow away from a heat source (e.g., an active device). Provision of a heat sink (sometimes also including a heat spreader) in thermal communication with the heat source is a typical method for cooling the heat source. There are many ways to provide thermal communication between heat source and heat sink. For example, the heat source and the heat sink can be in mechanical contact. Although making mechanical contact between source and sink is a simple method for providing thermal communication, it has severe practical disadvantages. In particular, the thermal resistance between source and sink can vary dramatically depending on the detailed properties of the surfaces making contact. Furthermore, it can be undesirably expensive to provide good thermal contact surfaces (i.e., clean, flat and smooth) on source and sink.
Accordingly, a commonly employed approach for providing thermal communication between heat source and heat sink is to position the source and sink in proximity, and fill the space between them with a thermal interface material (TIM). The TIM is typically a relatively viscous liquid or a flexible solid (i.e., it is mechanically compliant). A compliant TIM greatly reduces the flatness and smoothness requirements on the source and sink thermal contact surfaces, since the compliant TIM can flow or deform to make contact with irregular surfaces. Mechanically compliant TIMs are also highly useful to prevent formation of voids due to thermal cycling combined with thermal expansion mismatch. In addition to mechanical compliance, another desirable characteristic of a TIM is high thermal conductivity.
Various approaches for improving TIM thermal conductivity have been considered in the art. Several thermal interface materials are known which have inclusions in a matrix material, where the inclusions have higher thermal conductivity than the matrix material. For example, U.S. Pat. No. 6,311,769 considers a TIM having graphitized fiber inclusions arranged to protrude from the surface of the TIM. Such arrangement of fibers may not be easy to provide in practice, so approaches which require no special alignment of the inclusions are preferable. Another example is U.S. 2004/0209782, which considers carbon nanotube or carbon nanoparticle inclusions in a TIM. Carbon nanotubes and/or nanoparticles have also been considered for use in heat spreaders (e.g., as in U.S. Pat. No. 6,407,922). A heat spreader is typically a rigid solid structure for facilitating the lateral flow of heat (e.g., to reduce peak temperature at hot spots). Thus a heat spreader can be used in conjunction with a TIM, but is not itself a TIM. The function of a TIM is to facilitate the flow of heat across an interface separating source from sink (i.e., longitudinal heat flow as opposed to lateral heat flow), and this function is different than the function of a heat spreader. Carbon nanotubes (e.g., as in the preceding two examples) are promising materials for inclusion in TIMs, since their thermal conductivity is relatively large. However, this promise has not been completely realized thus far. One reason for this is that adding high concentrations of carbon nanotubes to a TIM frequently increases the TIM viscosity to undesirably high levels.
Accordingly, it would be an advance in the art to provide a thermal interface material having high thermal conductivity, relatively low nanofiber concentration and a relatively low viscosity.